This is Thomas Mayer's blog about vacuum tube audio, to share updates about new amplifiers and preamplifiers and ELROG vacuum tubes.

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Friday, November 29, 2013

Tube of the Month : The 6CJ3

Hi!

In the power supply for the recently presented UX-201-A circuit, I am using 866A mercury rectifiers along with 6CG3 TV damper tubes. In this months tube article I would like to present a suitable alternative, The 6CJ3.

JC Morrison was an early advocate of the use of TV dampers as rectifiers and also used the 6CJ3 in some projects which got presented in the famous Sound Practices magazine in the 1990ies.

For all technical parameters please see the General Electric data sheet. The 6CJ3 mainly differs in the base from the 6CG3. While the latter has a 12-pin compactron base, the 6CJ3 is a 9-pin Novar tube. The pinout is shown on the left. The Novar base is not to be confused with the Noval base which is also called miniature base. On the Novar base the pins are placed along a circle with a larger diameter. To make the confusion complete there is another 9-pin base called Magnoval.Novar and Magnoval bases are very similar, they only differ in the diameter of the pins. The Magnoval pins are a bit thicker. This means a 6CJ3 would slip easily into a Magnoval socket but would not get good contact. As mentioned the 6CJ3 is electrically very close to the 6CG3. They have the same heater voltage and current and similar peak inverse voltage ratings and they can deliver similar currents. They also look very much the same. Several times I tried to shove a 6CJ3 into the rectifier socket of an amp which was configured for 6CG3. Only realising after several attempts that somehow the number of pins did not match the number of holes in the socket. There had been a lot of TV damper types which were actually very close electrically. Probably the tube manufacturers wanted to have their own types for certain applications. More about that later on.

Here is a photo showing the 6CJ3 in comparison to the 6CG3:

Very similar in size, shape and internal construction. Only different bases:

And here a photo showing the difference between the Novar base of the 6CG3 and a Magnoval base, in this case of a EY500, a tube which will be covered in a future article:

The Novar is on the left and Magnoval on the right. Very easy to mix up, only a close look reveals the difference in pin diameter:

And finally a comparison between the Novar and Noval bases:

As mentioned above there had been an outburst of TV damper type registrations from different manufacturers for almost the same tubes. Later on it became common practice to print several type designations on tubes to cover that manifold of numbers with just a few tubes. For example like this:

This tube bears three type designations: 6CJ3, 6DW4B and 6CH3. Interestingly these types are close electrically but not identical. For example the 6DW4B data sheet specifies a significantly lower heater current of 1.2A versus the 1.8A of the 6CJ3. Also the current and voltage ratings differ a bit. While this might be ok for most applications it can become critical if a circuit relies on the exact values of a type.

Here is another example:

If you have any of these tubes with multiple type designations, best practice would be to assume the worst case ratings and make sure the circuit will work with them.